The invention relates to a steam-drive process for producing a viscous oil or tar. More particularly, the invention provides a process for recovering oil from a subterranean viscous oil reservoir, or at least a portion of such a reservoir, in which the stratification due to differences in the absolute permeabilities of the individual layers of the rocks is not sufficient to cause a significant portion of the injected steam to flow through only one or a few layers of exceptionally high permeability.
Numerous prior processes have been suggested for producing an oil or tar from such a relatively non-stratified reservoir. For example, the J. Van Lookeren patent No. U.S. Pat. No. 3,367,419 suggests injecting steam and producing fluid at laterally spaced locations and, after steam breakthrough, reducing the tendency for steam to bypass the production locations by producing from them at rates that reduce the pressure within the steam channel, to less than that in other portions of the reservoir. The R. W. Durie patent U.S. Pat. No. 3,439,742 suggests opening injection and production wells into the lower portion of a reservoir in which an oil zone of low effective permeability overlies a water zone of high permeability, then injecting steam and producing fluid in a manner that forms a steam channel (located along the upper part of the lower zone) between the injection and production locations. The R. W. Bowman patent U.S. Pat. No. 3,682,244 suggests that where an oil layer of low effective permeability overlies a water layer of high effective permeability, steam be injected and fluid produced at locations within the water layer and, whenever steam breakthrough becomes imminent at a production location, plugging back the bottom portion of the well at that location. U.S. Pat. No. 3,771,598 suggests injecting steam and producing fluid at spaced locations within a viscous oil reservoir and, when steam breaks through at the nearest production location, throttling that production well and increasing the injection pressure, then repeating such steps relative to each producer, to avoid heat losses (via casing vent gas) while increasing the temperature and pressure within the steam channel. U.S. Pat. No. 3,796,262 suggests injecting steam and producing fluid at laterally spaced locations within a viscous oil reservoir, injecting faster than producing so that the pressure and temperature in the reservoir is increased, and then terminating the injection while producing fast enough to reduce the reservoir pressure to at least the flash point of water in the reservoir. The K. H. Wang, M. Prats and D. A. Campbell patent U.S. Pat. No. 3,847,219 suggests injecting steam and producing fluid at laterally spaced locations within a reservoir in which an oil layer of low effective steam permeability overlies a water layer of high effective steam permeability until a steam channel extends between the injection and production locations, then throttling the production while maintaining a sufficient injection rate to attain and maintain a relatively high pressure during a heating cycle, and then producing the heated oil at an increased rate during a low depressurization of the heated zone. U.S. Pat. No. 3,948,323 suggests steam soaking a viscous oil reservoir by injecting steam mixed with noncondensable gas until the injectivity to that fluid is diminished, due to hot mobile oil being displaced into a cooler zone where it becomes less mobile and reduces the effective permeability to steam, then injecting hot noncondensable gas free of steam, repeating to the extent required to heat a radial zone of significant size, and then backflowing to to produce the heated oil.
Also, numerous prior processes have involved various uses of steam in conjunction with a surfactant. For example, U.S. Pat. No. 3,292,702 suggests a steam soak process in which an aqueous surfactant is injected ahead of the steam and is said to provide an increased injectivity during the steaming period and a greater rate of production during the backflow. U.S. Pat. No. 3,357,487 suggests injecting a solution of surfactant prior to or during a steam injection so that a band of the surfactant solution is displaced by the steam. U.S. Pat. No. 3,412,793 suggests that, in a relatively highly stratified reservoir in which considerable differences in the permeabilities of the different strata cause significant proportions of an injected gas to flow into only the more permeable strata, a steam soak or steam drive process for recovering oil is improved by temporarily plugging the more permeable strata with foam. And, the control of how long such plugs are effective is improved by using steam as the gas phase of the foam. About eight years after applying for the U.S. Pat. No. 3,412,793, the same inventor applied for U.S. Pat. No. 3,994,345 relating to an improvement relative to a steam soak process for recovering oil. The improvement seeks to maximize the duration of the plugging effects of such steam foam plugs, so backflowing well shortly after the steam is injected does not cause live steam to flow through the more permeable strata and return to the production well. The U.S. Pat. No. 3,994,345 suggests injecting steam, injecting a foamforming material, and backflowing the well so that the steam then contacts the foaming agent to form a steam-blocking foam plug within the more permeable strata.
The T. K. Smith, Y. C. Chiu, J. B. Lawson patent U.S. Pat. No. 3,707,193 discloses that in a steam foam used for well cleaning operations, the noncondensable gas component of a steam foam-forming mixture of steam, noncondensable gas and surfactant, significantly increases the density, viscosity and weight-supporting capacity of a column of steam foam.